The technical field is gas chromatography, and in particular, retention time locking and multidimensional gas chromatography.
Multidimensional chromatography is technique that employs more than one separation stage (phase). Multidimensional gas chromatography is typically performed by coupling more than one gas chromatography column in series. The different columns usually have different stationary phases. The different stationary phases employ different separation mechanisms resulting in increased separation between the components of the sample. The columns are selected so that the components of interest in the sample will be separated in either one or the other or in combination of the two columns.
In standard multidimensional gas chromatography, the entire sample is introduced into the first column. The sample flows through the first column where the initial separation takes place. The sample is then transferred directly into the second column. The transferred sample then flows through the second column where the second separation takes place. Finally, the sample then flows from the second column directly into the detector.
There are several variations of the standard technique. Most commonly, only a portion of the sample is transferred from the first column into the second column. This technique, known as xe2x80x9cheart cutting,xe2x80x9d is used to effect separations in especially complex mixtures. The portion transferred to the second column generally contains a much less complex subset mixture than the original sample. A less common variation of the standard method, known as splitting, directs a fraction of the sample exiting the first column into a detector and directs the remaining fraction into the second column. The main advantage of heart cutting is it allows the chromatographer to monitor the separation on the first column as well as the second column.
Comprehensive multidimensional gas chromatography (CMDGC) is another variation of the standard technique. CMDGC employs an additional step during the transfer of the sample between columns. The additional step periodically focuses and desorbs the sample at a transition stage between columns. The focusing-desorption of the sample is accomplished by thermal modulation of the sample at the transfer point between the columns. The sample is accumulated and xe2x80x9cfocusedxe2x80x9d at a point prior to the entrance of the second column. Focusing is usually accomplished by a cooling device that retains the sample. The focused sample is then heated in the desorption step, which accelerates a portion of the retained sample into the second column. The accelerated portion of the sample or xe2x80x9cdesorptionxe2x80x9d is performed at timed intervals. The focusing-desorption step has the effect of releasing concentrated pulses of sample into the carrier stream, thereby increasing separation and detectability in the second column.
The focusing-desorption step is computer controlled. The computer records the focus time and the desorption time. The focus time corresponds to the elution time for an analyte from the first column. The desorption time corresponds to the injection time of the analyte into the second column. Elution time and injection time allows the chromatographer to determine the elution time of the solutes from the first column as well as the elution time of the second column.
The typical output from a CMDGC is a three dimensional (3D) plot with axes corresponding to retention time on the first and second column (usually x and y-axes), and the detector response representing the z-axis. Alternately, the 3D plot may be collapsed into xe2x80x9cisoxe2x80x9d plots that represent a top-down (x and y-axes) view of the standard 3D plot.
With all types of multidimensional gas chromatography (GC), additional dimensions are possible with the addition of more columns or with detectors that provide multidimensional signals. Examples of multidimensional signal detectors include mass spectrometers, absorbance spectrometers, and emission spectrometers.
A disadvantage of standard and comprehensive multidimensional GC is that the retention times (the time it takes analytes to elute from a column) for single or multiple compounds can vary from instrument to instrument and even day to day on the same instrument. The variations, which can occur in each column of a multidimensional system, may be due to instrument drift, atmospheric changes, oven design, column dimension differences such as length or diameter, and stationary phase degradation.
The inconsistency of retention times increases the complexity of the resulting data. Inconsistency of retention times also disrupts the timing for heartcutting, thereby leading to inaccurate results. The data reduction and interpretation time resulting from these variations is increased significantly. The chromatographer must compensate for the variations or reanalyze the samples prior to interpreting the results each time a variation occurs. In effect, every data set containing a retention time variation must be evaluated as if it were a new method.
Retention-time locking is a technique that adjusts operational parameters of a gas chromatograph to avoid variations in retention time. Retention-time locking compensates for system, time-to-time, and location-to-location matching of retention times between known or reference systems and new or different systems.
Retention-time locking is accomplished through various methods. The only requirement of retention-time locking is that the columns used have the same stationary phase type (chemistry) and the same nominal phase ratio. Most commonly, the column head pressure on the new or different system is adjusted so that the column void time or the retention time of a known analyte equals a defined value (the defined value being ascertained on a reference system). Head pressure is most commonly regulated by a precise pressure controller. The adjusted head pressure compensates for differences in column and operational parameters producing retention times identical or nearly identical to those of the reference system.
Some varieties of pressure controllers can also react and adjust to changes in operating conditions including, for example, changes in ambient (atmospheric) pressure and temperature fluctuations. The added control can help to fine tune head pressure and provide even better retention time stability.
Retention time locking can be utilized in combination with other chromatographic techniques such as, for example, method translation and retention time factors. Method translation is a process that allows one to predictably scale a known set of chromatographic conditions in response to a desired change in one or more system parameters. Functions that relate gas flow rate in the column to column dimensions (length and diameter), temperature, carrier gas type, stationary phase film thickness, inlet pressure, and outlet pressure are used to calculate appropriate new sets of conditions. Using method translation, peak elution order and relative retention are maintained, and retention times of analytes are precisely predicted. Because there is usually some uncertainty in the exact column dimensions, oven temperature, and stationary film thickness, method translation can be followed by retention time locking to better match new retention times to a predicted retention time on a reference system.
Retention factors represent normalized retention times. Considering that GC methods can be scaled, reduced representations of retention time resulting from locked but scaled methods can be more easily compared or used. For example, results from a reference GC can be searched against the same library of retention factors for a scaled GC system that is running at fives times the speed of the reference system. If retention factors were not used, either the chromatographic data from the faster system would have to be multiplied by 5, or the data in the library would have to be divided by 5 prior to searching. The concept of using retention factors with retention time locked GC systems is highlighted in U.S. Pat. No. 6,153,438.
A method applies retention-time locking to multidimensional gas chromatography. Retention time locking is applied to both standard and comprehensive multidimensional gas chromatography. The method simplifies data interpretation compared to conventional methods. The consistency of retention times generated by the method allows users to reduce time spent correlating data generated over time and between instruments. The consistency of retention times also allows the creation of a general compound library or map that can be used for compound identification, compound class identification and determining the chemical nature and properties of sample components on any similar multidimensional GC system operated under locked conditions.
In an embodiment, retention time locking may be applied to either or both of the columns in a multidimensional gas chromatography system. Additionally, if the multidimensional system contains more than two columns in series, retention time locking can be applied to any or all of the columns as required.